This invention is directed at thermally insulating the heart during an open heart surgical procedure. The device reduces heat gain of the heart from its surroundings and further serves to protect the phrenic nerves from damage due to being exposed to low temperatures.
Surgical repair of occluded or semi-occluded coronary arteries by coronary artery bypass grafting, or repair or replacement of cardiac valves, as well as some other cardiac surgery is undertaken in conjunction with a cardio-pulmonary bypass procedure. During the period of cardio-pulmonary bypass the pumping function of the heart and the gas transfer function of the lungs are temporarily taken over by a heart-lung machine. During the period of cardiac surgery the myocardium is preserved by cooling of the heart to a temperature in the range of approximately 5.degree.-15.degree. Celsius. This cardiac cooling may be accomplished by substituting the normal coronary artery perfusion of warm blood by the infusion of a cold cardioplegia solution into the aortic root. In addition, the core temperature of the patient is usually lowered to 26.degree.-28.degree. C. to help reduce the temperature of the heart. The patient's body temperature is decreased (and subsequently raised) by means of a heat exchanger in the heart-lung machine. Topical cooling of the heart may also be used to reduce myocardial temperature. This may be accomplished by placing soft ice slush or cold saline solution on or around the heart, or by enclosing or partially enclosing the heart with a recirculating type cardiac cooler, such as are described in U.S. Pat. Nos. 4,154,245 and 4,259,961. Profound cardiac hypothermia is maintained for the duration of aortic cross clamping because various studies have shown that cardiac cooling has reduced the incidence of operative mortality. To maintain the temperature of the myocardium in the range of about 5.degree.-15.degree. C., it is necessary to reduce cardiac heat gain to a low level. Internal cardiac heat gain is minimized by preventing the perfusion of the heart with warm blood by cross-clamping the aorta distal to the coronary ostia. In some patients an extra coronary blood supply is thought to be by way of bronchial artery collaterals.
There are four possible sources of external cardiac heat gain. Firstly from the tissues surrounding the heart in the pericardial cavity. Usually this heat gain is reduced by lowering the patient's body temperature to approximately 26.degree.-28.degree. C., hence reducing heat flow into the heart. Secondly, from the air adjacent to the heart. Thirdly, from the surgeon's hands, and lastly from radiant heat from the operating room lights. Topical cooling has been used to reduce myocardial temperatures, and help negate the effects of heat gain from the four sources mentioned above. However, occasionally, the use of topical cooling by ice or a recirculating jacket has caused thermal damage to the phrenic nerves (which lie immediately adjacent to the lateral sides of the pericardial cavity). A cardiac insulator should thus both thermally insulate the heart and also offer thermal protection to the phrenic nerves. Effective thermal insulation of the myocardium from the chest cavity can also obviate the necessity for reducing the body temperature of the patient. Hence operative time spent during cooling and rewarming is eliminated, potentially reducing overall time of operation.
Cardiac insulators are known in the art. For example Shiley Laboratories, Inc., Santa Ana, California have such a pad in commercial distribution, and an experimental device (incorporating a cardiac insulator) was described by Daily, P. O.; Pfeffer, T. A.; Wisniewski, J. B.; Steinke, T. A.; Kinney, T. B.; Moores, W. Y.; and Dembitsky, W. P., Clinical comparisons of methods of myocardial protection, J. Thorac Cardiovasc Surg. Volume No. 93, 1987 pp. 324-336.
Although these devices, disclose the general principle of providing thermal insulation between the myocardium and the pericardial cavity, their particular construction creates difficulties in use. Namely, the former has several disadvantages. It is of such inappropriate shape that only part of the posterior and lateral portions and none of the anterior portion of the myocardium is insulated, and the phrenic nerves are not fully protected. Moreover the pad, being fully flexible, is difficult to insert under the adult heart, and has a marked tendency to slip around the pericardial cavity in use.
The cooling jacket described by Daily et al. (and another described in U.S. Pat. No. 4,154,245) incorporates a thin layer of insulating material, and a malleable skeleton, of different form and function to the semi-rigid stiffner used in this invention. In the Daily device the malleable metal skeleton allows the pad to be shaped around a portion of the diameter of the ventricular surfaces. Moreover the device described by Daily is not intended solely to act as a cardiac insulator, nor for use with soft ice slush or a cold saline solution applied as topical cardiac cooling, and as such offers no thermal protection of the phrenic nerves.
The semi-rigid stiffner used in the preferred embodiment of this invention is used to enable the device to be placed under the posterior myocardium, and retained in the appropriate anatomical location.
It is an objective of this invention to provide a myocardial thermal insulator.
It is a further objective of this invention to provide a thermal insulator to insulate the posterior, and lateral surfaces of the myocardium from the pericardial cavity.
It is a further objective of this invention to provide a thermal insulator to thermally protect the phrenic nerves from hypothermal injury due to topical cooling of the heart by soft ice slush, cold saline solution, or a recirculating type cardiac cooler. In the preferred embodiment of this invention it is a further objective of this invention to provide a myocardial thermal insulator containing a semi-rigid, malleable member which allows the surgeon easily to insert the device between the heart and the pericardial cavity.
It is a further objective of this invention to provide a thermal insulator to insulate the entire myocardium, and to thermally protect the phrenic nerves from hypothermal injury.
It is a further objective of this invention to provide a myocardial thermal insulator which may easily be inserted under the heart.
It is a further objective of this invention to provide a myocardial thermal insulator which will be retained in place under the heart.
It is still a further objective of this invention to provide a myocardial thermal insulator which will be retained in place around the heart.
It is still a further objective of this invention to provide a myocardial thermal insulator which will obviate the necessity of having to reduce the body temperature of the patient, hence potentially shortening overall time of the operation.
The simplicity and design of the cardiac insulator are such that it is quite inexpensive and hence may be disposable, hereby eliminating the necessity of cleaning and sterilizing between uses.
Other objects and advantages of this invention will be more apparent from the detailed description of the device which follows.